![]() ![]() The biometeorological science of phenology has developed over the past five decades, with methodologies to determine the climate drivers responsible for phenological shifts. While factors such as soil moisture, temperature and composition, sunshine hours, and the health of the tree can affect the mean flowering date, the shifts in flowering are driven by climate. That dormant period often requires a certain number of days below a threshold temperature, or an accumulation of chilling units.įor some plants, the onset of rainfall is also important in triggering blossoms. Spring blossoms are triggered, in most cases, by temperatures warming above a certain threshold, following the completion of a dormant period. ![]() While phenological shifts are highly species and location specific, the broad climate drivers are well understood. At a critical threshold, the flowering season will become unsuccessful. Therefore, although phenological shifts represent an adaptation in plants and animals, these advances in flowering dates cannot continue indefinitely. If plants flower too early in the year, they are at risk of frost damage during the late winter months, and often do not complete their dormancy. Rainfall changes during this time were less uniform. The advance in flowering took place against a backdrop of warming temperatures, ranging from 0.1-0.2☌ per decade for daily maximum temperatures and a more rapid 0.2-0.4☌ per decade for daily minimum temperatures. We then explored the climatic drivers of this advance, by comparing the flowering dates to meteorological data from across Gauteng. The records confirmed the advance in flowering dates, and from these we quantified a mean rate of advance of 2.1 days per decade. This record allowed us the chance to contribute to the global attempts at recording phenological shifts. We mined these sources to compile a list of flowering dates of jacaranda trees spanning 1927-2019. There is very little phenological data for South Africa, and so very little phenological research has been conducted compared to the work in countries across Europe, Asia and North America.īecause jacaranda blossoms result in such a dramatic change in the urban landscape each year, they are often reported on in the news and, more recently, in social media posts. Phenological shifts are species and location specific – in some areas, and for some species, events are even being delayed as a result of specific climate drivers. We explored this change in the timing of jacaranda blossom in our paper published in the Journal of Urban Forestry and Urban Greening. Phenological analyses show that current cherry blossoming is occurring earlier than any time in the last 1,200 years. Not only are the cherry blossoms a key tourist attraction, and the cherry festivals important cultural events, but this also represents the world’s longest phenological record. The most notable example is the Japanese cherry blossoms. This is referred to as a phenological shift, and is being observed across a range of species globally as a result of climate change.Įxplainer: why phenology is key in tracking climate change Gradually over the decades, the date of bloom has advanced through October to the early weeks of September. In the 1920s and 1930s, the trees only started to bloom in mid-November. Octogenarian residents who have lived in Gauteng their whole life might remember that jacarandas did not always flower in September. Jacarandas were introduced to Pretoria and later Johannesburg in the early 1800s, specifically as ornamentals to line the streets of the suburbs and central business districts. ![]() Johannesburg alone is recorded to have over 10 million trees. The cities of Johannesburg and Pretoria are well covered with trees – and jacarandas ( Jacaranda mimosifolia), with their purple blooms in late spring, are a prominent part of this urban forest.Ībout 16% of the land in the Gauteng City Region is planted with trees, forming one of the world’s largest and most densely vegetated man-made urban forests. In September each year, South Africa’s Gauteng province turns purple. ![]()
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